1. Field of the Invention
This invention relates to the field of integrated circuit (I.C.) leadframes, and particularly to leadframes designed to provide improved heat dissipation and/or increased maximum die size.
2. Description of the Related Art
A I.C. package's "leadframe" includes the leads that extend from the package, the structure that supports the I.C. die, and any structures employed to hold these elements together. Many types of leadframes have been developed and are in use, including "lead-under-paddle" (LUP) and "chip-on-lead" (COL) leadframes, for example.
Plan view and corresponding cross-sectional views of a standard 8-lead "mini small outline package" (MSOP) leadframe are shown in FIGS. 1a and 1b, respectively. As used herein, a "standard" package is one for which a set of dimensions has been defined and adopted by the I.C. industry. An I.C. die 10 is on a "paddle" 12, and is secured to the paddle with some type of adhesive 14. The ends of the package's leads 16 which are to be connected to the die are arrayed around, but physically separate from, the paddle 12, and the interconnections between the die and the leads are provided by wire bonds 18. The ends of the leads that will be encapsulated within the package are referred to herein as the "inner" portions of the leads, with the portions that extend out from the package referred to as the "outer" portions.
When operating, an I.C. die generates heat, and many leadframes include features to aid in dissipating the heat produced by the die. For example, in the leadframe of FIG. 1, the paddle 12 is often made of metal, and the adhesive 14 is thermally conductive, so that some of the heat generated by the die is conducted to the paddle. The physically separated paddle 12 and leads 16 are held in a desired proximity to each other with a mold compound 20 (not shown in FIG. 1a for clarity), which forms the exterior of the I.C. package 22. When heat is generated by I.C. die 10, it is conducted to the paddle as noted above, and then to the leads 16 through the mold compound 20. Unfortunately, molding compounds in common use have a very low thermal conductivity. This increases the thermal resistance between the die and the leads, and limits the amount of heat that can be dissipated by the die. The limited heat dissipation capabilities of such a package in turn limit the power that can be consumed by the I.C. die, as well as the uses to which the die can be put. Poor heat dissipation can also accelerate the failure rate of the die's circuitry.
Wire bonds are usually "wedge bonded" to the package leads, typically accomplished by pressing a gold wire bond to a small silver-plated area (the "wedge bond shelf") of a copper lead. However, to provide good anchoring of the package leads, the inner portions which include the bond shelves should extend well into the molding. This reduces the maximum size of the paddle--and consequently the maximum die size--i.e., the die's surface area--that can be accommodated in a given standard package size.
One approach to these problems is found in U.S. Pat. No. 5,541,446 to Kierse. Here, an I.C. die is affixed directly to the package leads, and the paddle is eliminated. This arrangement helps to reduce the package's thermal resistance, but does nothing to address the die size area lost to the bond shelves. Furthermore, with the package encapsulating only the leads and the die, it may be difficult to ensure the coplanarity of the package leads.